Sáenz was born in 1954 in Old Picacho, a small farming village outside of Las Cruces, New Mexico, forty-two miles north of the U.S. / Mexico border. He was the fourth of seven children and was brought up in a traditional Mexican-American Catholic family. He entered the seminary in 1972, a decision that was as much political as it was religious. After concluding his theological studies at the University of Louvain, he was ordained a Catholic priest. Three and a half years later, he left the priesthood.

At the age of 30, he entered the University of Texas at El Paso. He later received a fellowship at the University of Iowa. In 1988, he received a Wallace E. Stegner Fellowship in poetry from Stanford University. In 1993, he returned to the border to teach in the bilingual MFA program at UTEP.

His 2012 novel Aristotle and Dante Discover the Secrets of the Universe (Simon and Schuster) is an Honor Book for the Printz Award, and the winner of the Pura Belpré and the Stonewall Awards.

He is the author of numerous novels, books for children and young adults. His beloved young adult novel Sammy & Juliana in Hollywood, a staple in many high school curricula, was a finalist for the L.A. Times Book Prize, a Top Ten Best Book for Young Adults, and winner of the Américas Award.

Sáenz is the author of a previous book of poetry, Calendar of Dust, which won an American Book Award. Cinco Puntos published two of his other books of poetry called Elegies in Blue and the now out of print, Dark and Perfect Angels.